1. Field of the Invention
The present invention relates to a hydraulic servo regulating systems of frictional engaging means in an automatic transmission of an automobile, particularly to a regulating system comprising regulator valves, etc., which are disposed in oil paths extending between shift valves, hydraulic servos and accumulators.
2. Description of the Prior Art
An automatic transmission, in general, has a gear mechanism such as a planetary gear mechanism whose components are engaged or restrained by clutches or brakes (frictional engaging means) to obtain a desired speed. These frictional engaging means are conrolled by hydraulic servos to which line pressure is applied or drained by the shifting of the shift valves. Each hydraulic servo has an accumulator, and line pressure is applied from the shift valve to the accumulator through an orifice. Due to the above structure, a surge in the line pressure applied to the hydraulic servo is prevented, and the degree of frictional engagement is determined based on a hydraulic characteristic of the accumulator.
The hydraulic servo is rotating when the clutch is engaged. At this time the oil is subjected to a centrifugal force as it is drained from the servo. Centrifugal force is effective in the servo to create residual hydraulic pressure. To release the residual hydraulic pressure, a check valve for draining oil under the centrifugal force is incorporated in a piston. Consequently, in the hydraulic servo having the check valve, a rise in line pressure is not attained because oil leaks from the check valve unless higher line pressure is generated which is sufficient to overcome the centrifugal force. For the above reasons, in the case of the prior systems which apply line pressure to the hydraulic servo through an orifice, a malfunction may occur because oil is supplied rather slowly and the volume is comparatively small, so that enough line pressure to overcome the centrifugal force cannot be generated. In addition to the above, to obtain the necessary degree of frictional engagement, an accumulator having a sufficient capacity is required because the degree of frictional engagement is determined by the hydraulic characteristics of the accumulator. This hampers the minimization of the accumulator contrary to the desirability of compact automatic transmissions.
Furthermore, when the frictional engaging means is released, oil in the accumulator and the hydraulic servo flows through a check valve disposed in parallel with the orifice. The oil is drained through the shift valve. Due to this, a path through which oil is drained is relatively long. Accordingly, a draining efficiency is diminished because of pipe-resistance contributing to relatively poor performance of the releasing of the frictional engaging means.